Semiconductor diode lasers are formed of multiple layers of semiconductor materials. The typical semiconductor diode laser includes an n-type layer, a p-type layer and an undoped active region layer between them such that when the diode is forward biased electrons and holes recombine in the active region layer with the resulting emission of light. The layers surrounding the active layer typically have a lower index of refraction than the active layer and form cladding layers that confine the emitted light to the active layer and sometimes to adjacent layers. Semiconductor lasers may be constructed to be either edge emitting or surface emitting. In an edge emitting Fabry-Perot type semiconductor laser, crystal facet mirrors are located at opposite edges of the multi-layer structure to provide reflection of the emitted light back and forth in a longitudinal direction, generally in the plane of the layers, to provide lasing action and emission of laser light from one of the facets. Another type of device, which may be designed to be either edge emitting or surface emitting, utilizes distributed feedback structures rather then conventional facets or mirrors, providing feedback for lasing as a result of backward Bragg scattering from periodic variations of the refractive index or the gain or both of the semiconductor laser structure.
Semiconductor lasers having continuous wave (CW) power in the watt-range and narrow bandwidth, e.g., less than 2 Å full width half maximum (FWHM), would be desirable for a variety of applications. Examples include 0.894 μm diode lasers which may be used for polarizing Cs to generate spin-polarized Xe gas for magnetic resonance imaging, low-chirp pump sources for solid state lasers, and infrared spectroscopy sources for monitoring environmental gases. Conventional broad stripe (≧25 μm) semiconductor lasers used for obtaining high powers typically have a spectral width of about 20 Å FWHM or more at high drive levels and broaden further under quasi-CW operation. Significant improvements in spectral width can be obtained using distributed feedback (DFB) gratings or distributed Bragg reflectors (DBR) rather than Fabry-Perot mirror facets for optical feedback. For applications where (lateral) spatial coherence is not necessary, a broad-stripe laser with a DFB grating is apparently well suited for achieving high CW powers with narrow spectral linewidth.
A limitation is encountered with DFB lasers designed to operate at shorter wavelengths, including visible light wavelengths, in that conventional diode lasers grown on GaAs substrates, which can emit in the range of wavelengths between about 0.6 μm to 1.1 μm, generally have optical confinement layers next to the active region that contain aluminum, as well as cladding layers containing aluminum. Due to the high reactivity of aluminum (i.e., essentially instant oxidation when exposed to air), it has proven to be very difficult to make single frequency lasers of the DFB type in the foregoing wavelength range in which the grating is buried within the multi-layer semiconductor structure. Consequently, the commercially available high power, narrow linewidth lasers have been of the distributed Bragg reflector (DBR) type, in which the grating is outside of the active lasing part of the structure. However, such DBR devices suffer from the major drawback of mode hopping that occurs with increasing drive current due to changes in the lasing-region index of refraction with increasing drive power. As described in U.S. Pat. No. 6,195,381, improved high power edge emitting semiconductor lasers can be formed with a distributed feedback grating in an aluminum free section of the upper confinement layer to act upon the light generated in the active region to produce lasing action and emission of light from an edge. Such devices are well suited to be formed to provide a wide stripe and high power, in the one watt range, at various wavelengths including visible wavelengths.
A major objective in the development of high power lasers is improvement of the wallplug efficiency, that is, the light output relative to the electrical power input. See D. Botez, et al., “66% CW Wallplug Efficiency from Al-free 0.98 μm-emitting Diode Lasers,” Electronics Letters, Vol. 32, No. 21, 10 Oct. 1996, pp. 2012-2013. Separate confinement heterostructure (SCH) semiconductor laser structures having relatively thick (greater than 0.5 μm) optical confinement layers display large built-in voltages, V0, due to the non-ohmic voltage drop, ΔVno, which in large part is due to slow carrier transport, especially injected holes, in the confinement layers. ΔVno is equal to Vo−Vf, where Vf is the quasi-Fermi level difference in the active region. The non-ohmic voltage drop ΔVno is a significant factor in the overall wallplug efficiency of the laser.
Another problem is encountered in SCH structures when the quantum-well active region is placed close to the n-doped cladding layer. An asymmetric SCH structure of this type is necessary for a large transverse spot size structure and especially for high power ARROW-type devices. See U.S. Pat. No. 6,167,073. In a diode laser having an asymmetric type structure, low mobility injected holes from the p-doped cladding layer which have a small diffusion length can result in poor overall injection efficiency.